Field of the Invention
The present invention relate to an improvement in a unifocal optical imaging system used in various kinds of camera to form an image of a subject, especially an optical imaging system of an imaging device using an electric imaging unit such as a digital camera or a digital video camera. It also relates to a camera and an in-vehicle camera incorporating such an optical imaging system.
Description of the Related Art
Among imaging devices including an optical imaging system and an area sensor, a security camera, a monitoring camera for a manufacturing line, and an in-vehicle camera have been increased in demand. These cameras are used for sensing a target object. For example, an in-vehicle camera is required to recognize the position of a running vehicle or the state of a road surface. In view of this, an image forming lens of such a camera needs to exert high resolution for the purpose of recognizing relatively small objects or precisely observing a situation in a relatively far location. Further, in order to accurately acquire information of a target object including size and shape, the image forming lens with a wide angle of view is required to optically correct distortion appropriately. In addition to high resolution and low distortion, it has to be a large diameter lens with small F-number to be able to properly recognize an object even in a dark ambience as night. In general an imaging device for sensing is preferably small in size to be unnoticeable. Moreover, for in-vehicle or monitoring use, an optical imaging system is preferably resistant to a thermal change or a change in usage environment since it is likely used outside.
Japanese Patent No. 4667269 (Reference 1) and No. 5418745 (Reference 2), and Japanese Laid-open Patent Application Publication No. 2012-220741 (Reference 3) disclose retrofocus imaging lenses for sensing in which filters including a lowpass filter and an infrared cut filter are arranged in a space between an optical system and an image sensor. However, the imaging lens in Reference 1 does not secure sufficient F-number despite its good lens performance and low distortion. In addition, it cannot acquire sufficient angle of view with a small-size sensor due to a long focal length and it is not sufficiently compact. Likewise, the imaging lens in Reference 2 realizes low distortion but cannot acquire sufficient angle of view with a small-size sensor due to a long focal length and it is not sufficiently compact either. The imaging lens in Reference 3 is compact-size and has excellent environment resistance; however, it cannot secure back focusing and realize low distortion sufficiently.